1. Field of the Invention
The present invention relates to an electrode wire used as a connection lead wire for a solar cell.
2. Description of the Related Art
A solar cell includes a semiconductor substrate of a silicon semiconductor having PN junctions, and a connection lead wire soldered to a solder band provided across a plurality of linear front surface electrodes disposed on a front surface of the semiconductor substrate. A plurality of such solar cells are generally connected in series to provide a desired electromotive force. For the serial connection, one surface (a lower surface) of a connection lead wire is soldered to a front surface electrode of one solar cell, and the other surface (an upper surface) of the connection lead wire is soldered to a relatively large rear surface electrode of an adjacent solar cell.
An electrode wire that is conventionally used as a material for the connection lead wire includes a core material of a flattened copper wire prepared by rolling a tough pitch copper wire having a round cross section, and hot-dip solder plating layers formed on surfaces of the core material. The formation of the hot-dip solder plating layers is achieved by using a hot-dip plating method for plating the flattened copper wire. That is, the surfaces of the flattened copper wire are cleaned with an acid or other suitable cleaner, and the flattened copper wire is dipped in a molten solder bath, whereby the solder plating layers are formed on the surfaces of the core material of the flattened copper wire. The hot-dip solder plating layers each have a mound shape that bulges from lateral edges to a middle portion of the core material by surface tension occurring when the molten solder adhering to the core material is solidified.
When the electrode wire is soldered to the semiconductor substrate, the heating temperature is strictly controlled at a lower temperature around the melting point of the solder. This is because there is a difference in thermal expansion coefficient between copper for the core material of the electrode wire and silicon as an exemplary material for the semiconductor substrate. That is, the electrode wire is soldered at a lower temperature in order to minimize thermal stress which may otherwise cause cracks in the expensive semiconductor substrate.
In a conventional case, the semiconductor substrate has a thickness of about 300 μm. However, in recent years, the semiconductor substrate tends to have a reduced thickness for cost reduction. A semiconductor substrate recently used has a thickness of about 250 μm. Therefore, the electrode wire using the conventional flattened wire as the core material is likely to cause cracks in the semiconductor substrate during the soldering. To prevent the cracking, an electrically conductive material having a smaller difference in thermal expansion from the semiconductor substrate material has recently been used for the core material. In JP-A-60-15937 (Patent Document 1), for example, a clad material is disclosed as the electrically conductive material, which includes an interlayer of Invar or an alloy of Fe and Ni (having a typical composition of Fe-36% Ni) and copper layers disposed on opposite surfaces of the interlayer. Besides Invar, Kovar (registered trade mark) or an Fe—Ni—Co alloy is used as the lower thermal expansion alloy.
In JP-A-59-204547 (Patent Document 2) and JP-A-59-204548 (Patent Document 3) , aluminum-copper based clad materials each including a chromium layer or a zinc layer formed in a bonding interface between an aluminum or aluminum alloy material and a copper or copper alloy material are proposed as a lead frame material for a semiconductor device, although they are used in a field different from the field of the solar cells.
The electrode wire (which is sometimes referred to as “clad electrode wire”) including the clad material as the core material as disclosed in Patent Document 1 alleviates the thermal stress occurring in the semiconductor substrate, but has an increased average electric resistance, which reduces the power generation efficiency of the solar cell, because the interlayer is made of an alloy material such as the Fe—Ni alloy or the Fe—Ni—Co alloy which has a relatively high volume resistivity.
The aluminum-copper based clad materials of Patent Documents 2 and 3 are used in a different field from the field of the solar cell electrode wire. In addition, aluminum is exposed on one surface of such a clad material, making it impossible to form the hot-dip solder plating layer on the surface.